swith gb2 assignment_groups ajax call to api

fixes CNVS-10367

Test Plan:
  - As teacher, make assignment groups with drop rules
  - Go to Gb2
  - Change weights of assignment groups and assignment grades
  - Ensure that proper assignment is dropped when grade is recalculated
  - Note: the headers will not properly change with new weight, this is
    being taken care of in another ticket

Change-Id: I194526211103515d8f5b5bfbdcaba2ee4ac2cb54
Reviewed-on: https://gerrit.instructure.com/28358
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>